The present invention relates to transmission of serial digital video, and more particularly to a scrambling system for serial digital video which inhibits the generation of pathological serial digital signals by dithering the least significant bit of certain video words only when the pathological serial digital signal would otherwise occur.
The standard for transmission of serial digital video is Society of Motion Picture and Television Engineers (SMPTE) Standard 259M. This standard specifies the use of a nine-bit scrambler followed by a non-return to zero (NRZ) to non-return to zero inverted (NRZI) conversion.
There are two classes of scrambling systems: synchronized and self synchronized. A synchronized scrambling and descrambling system has a pair of pseudorandom sequence generators, one at the transmitter end and the other at the receiver end. These pseudorandom sequence generators need to be periodically initialized or reset so that they are producing the same sequence with respect to the transmitted data.
A self synchronized scrambling and descrambling system does not have a separate pseudorandom sequence generator. Rather the transmitted data itself passes through a linear feedback shift register. Self synchronized scramblers and descramblers have the advantage that they do not need to be initialized or periodically reset. The self synchronized scrambler works by feedback, and the self synchronized descrambler works by feed forward. The shift registers in the scrambler and descrambler contain the same data, although the descrambler's data is time delayed. As soon as the registers in both the transmitter and receiver have filled with data, the scrambling and descrambling system is synchronized and the received data is valid. SMPTE 259M uses a self synchronized scrambler.
Although self synchronized scramblers are attractive in that no synchronization is required, they also have a serious flaw: pathological signals. Pathological signals are described in the paper Pathological Check Codes for Serial Digital Interface Systems by Takeo Eguchi, published in the August 1992 issue of The SMPTE Journal, incorporated herein by reference. Certain perfectly legal component video inputs, such as flat color fields, produce some highly un-scrambled looking bit sequences. It is possible to produce long runs of consecutive "ones" or "zeros" with very few transitions. It also is possible to produce extremely long periods with highly lopsided distributions of "ones" and "zeros", resulting in a large low frequency component. The pathological signals described by Eguchi include a repeating sequence of twenty "ones" followed by twenty "zeros", and another consisting of nineteen "zeros" and one "one". These patterns may repeat for the entire active video portion of a video line.
In a synchronized scrambling system it is possible to invent a data sequence which, when convolved with the scrambling sequence, produces long runs of the same symbol and few transitions. In a well designed system such a sequence is extremely unlikely and the likelihood of its occurrence generally may be ignored because the data sequence itself is pseudorandom in nature, but very long and therefore vanishingly unlikely. But with the self synchronized scrambler, the data sequences which produce repetitive patterns with very few transitions are not pseudorandom, but rather represent flat color fields, such as might be produced by graphics generators. Therefore the likelihood that such "pathological" signals might actually be produced in normal system use is significant and cannot be ignored.
As a result a great deal of effort is expended on the design, manufacture and testing of cable equalizers and analog phase lock loops associated with serial digital video. Design, adjustment and operation of these circuits is quite critical because of the possibility of pathological signals. Pathological signals thus are the unfortunate result of the selection of a self synchronized scrambler architecture. The existence of a large amount of equipment makes it impractical to make an incompatible architectural change to the scrambling algorithm defined by SMPTE 259M.
What is desired is a scrambling system for serial digital video that is compatible with the current self synchronized architecture while eliminating the problem presented by pathological signals.